The search for alternative means of producing electrical energy has created significant interest in the formation of semiconductor devices that are capable of being economically manufactured and that can convert solar energy to electrical energy. Such devices should effectively collect incident light, efficiently convert the light energy to electrical energy and the devices should be capable of being produced at a low unit cost.
Prior efforts in development of such devices has generally been directed to two different forms of semiconductor material. The first is a single crystal material that is doped to produce a semiconductor solar cell. Such cells are relatively efficient, exhibiting efficiencies of approximately 14-16%; however, they are difficult to economically manufacture on a large scale because they rely on the casting of high purity single crystals. While the technology exists to manufacture solar cells of such a type, the batch casting process and the difficulties in maintaining purity of such castings makes such a process inherently difficult and uneconomic.
A second type of material is produced in a different way to form thin polycrystalline films of semiconducting materials which in turn may form a heterojunction solar energy transducer. The method of making this second type of device comprises the present invention.
Such devices are well known and have been the subject of numerous publications. U.S. Pat. No. 4,335,266 to Mickelsen and Chen discloses and claims such a device. Generally, it discloses means for adjusting the copper/indium ratio in the chalcopyrite portion of a CdS/CuInSe.sub.2 heterojunction and the articles made by the disclosed technique. That patent also describes and cites a number of references that disclose the general state of the art in this technology.
The previous Mickelsen and Chen patent discloses a method of making both semiconductor layers of a heterojunction device. The present invention is, however, directed primarily to a method for forming one portion of a layered semiconductor device, e.g., the n-type Zn.sub.x Cd.sub.1-x S portion of a Zn.sub.x Cd.sub.1-x S/CuInSe.sub.2 n-p heterojunction cell. The invention also includes articles made by the unique forming technique.
Previous attempts to form mixed metal compound films such as Zn.sub.x Cd.sub.1-x S resulted in unacceptably high film resistances, especially at Zn contents of in excess of 15%. In order to reduce the resistivity of the film, the temperature of the vapor source was increased but this in turn affected the deposition rate, film composition, and the chamber pressure. In such a technique, the two separate vapors were generated in different portions of a multicomponent vapor source and then mixed prior to introduction to the deposition process by means of a common orifice. The vapor source was heated to a single temperature and the rate of vapor production of the individual vapors was controlled primarily by the orifice sizes of the various portions of the vapor source.
The shortcomings of such a technique were eliminated by practice of the present invention, which provides a relatively inexpensive method of making a mixed metal compound in the form of a thin film having properties much improved over previous processes. Specifically, the present invention provides a means for economically producing a mixed metal compound heterojunction solar cell having excellent conversion efficiencies.